1. Field of the Invention
This invention generally relates to pulleys. More particularly, the present invention relates to an overrunning clutch pulley which has particular application within an engine accessory system including an automotive alternator.
2. Description of the Prior Art
During operation of an engine, a belt drive system is sometimes used to power and operate the various accessory devices including, but not limited to, an alternator which provides electrical power to the vehicle. While several types of belt drive systems are in use, the system which is currently in favor is known as a serpentine drive system. Serpentine drive systems generally include a drive pulley connected to the crank shaft of the automobile's internal combustion (I.C.) engine and a ribbed belt trained about the drive pulley. The belt is also trained about one or more driven pulleys which are in turn connected to the input shafts of the various accessories. An automatic belt tensioner is also provided to maintain the tension of the belt within the proper range.
Most driven pulleys are provided in a one-piece design. These pulleys have no over-running capabilities, meaning that the pulley is rigidly mounted to rotate with the accessory input shaft. When the input shaft of the accessory device is running at high speed (up to 22,000 rpm for an alternator), a significant amount of inertia is built up within the accessory device. As a result of the combined inertia and the lack of over-running capabilities, relative slipping between the pulley and the belt can occur if the belt decelerates too quickly. If significant slipping of the belt occurs, an audible squeal will be produced. Not only is a squealing belt annoying from an auditory standpoint, but it also undesirable from a mechanical standpoint since it produces undue wear on the belt itself.
In a typical driving situation, the accessory belt will experience instances of large deceleration, such as during in a 1-2 upshift during wide open throttle acceleration. The situation is made even worse if the throttle is closed or "backed out" immediately after the transmission has been shifted. In these situations, the belt decelerates very quickly while the pulley, with the high inertia from the accessory, keeps rotating very quickly in spite of the friction between the pulley and the belt.
In attempting to cure the slipping of the belt and its associated problems, various design proposals have been put forward. One proposed design includes the use of higher belt tensions. However, in these designs the belt tends to wear even more quickly. Additionally, the bearings associated with the input shaft of the accessory also have shown greater wear and a shortened useful life. Various other tensioner constructions have been proposed, but those tensioners have generally exhibited poor performance and were costly to implement.
Attempts to cure the belt slippage and squeal problem have also proceeded in another direction. While early driven pulleys were constructed in a one-piece design, newer pulley designs have been proposed where the driven pulley itself exhibits an "over-running" capability. This allows the driven pulley to rotate in one direction relative to the input shaft of the accessory and therefore accommodate the inertia built up within the accessory.
U.S. Pat. No. 4,725,259 issued to Miyata discloses a construction where the driven pulley is mounted to the input shaft via a one-way clutch. The clutch only engages when the angular velocity of the pulley is increasing. Otherwise, the clutch slips relative to the input shaft of the accessory. This design is intended to smooth out the recurring fluctuations of instantaneous velocity in the belt that is typical of an I.C. engine. The design smooths out the corresponding recurrent instantaneous slipping of the belt relative to the driven pulley. No specific constructions for the Miyata one-way clutch are given in the disclosure of this patent.
U.S. Pat. No. 5,139,463 issued to Bytzek et al. discloses an alternator pulley construction in which a coil spring is disposed in a space between a hub attached directly to the accessory input shaft and a pulley mounted for relative movement exteriorly of the hub. The two ends of the coil spring are respectively bent radially inward and radially outward so that one will engage the hub and the other will engage the pulley. In this patent, the spring is wound so that when a positive torque is applied from the belt to the pulley, the rotational movement of the pulley will be transferred to the input shaft of the accessory as a result of the spring "winding-up" and the tangs engaging both the hub and pulley. Whenever negative torque is provided from the belt to the pulley, the spring enables the input shaft from the accessory and the hub to rotate relative to the pulley.
U.S. Pat. No. 5,156,573, also issued to Bytzek et al., discloses another clutch pulley construction where a coil spring is located in a space between a hub (attached to the accessory input shaft) and a pulley (mounted for relative movement exteriorly of the hub). Like the '463 patent, the coil spring in this patent also has one end bent radially outward. Additionally, the spring includes two sets of volutes, an intermediate set located between the other set and the bent end of the spring. The diameter of the hub and the inner diameter of the non-intermediate volutes are such that the volutes engage the hub when the pulley is being driven by the belt. When a negative torque is established between the alternator pulley and the input shaft, the volutes loosen with respect to the hub and allow slipping to occur. Importantly, the Bytzek design of the intermediate volutes allows for a "resilient rotational motion" to ease the shock loading between the pulley and the hub but can cause fatigue problems in the spring.
While the clutch pulleys of these patents may operate adequately in some respects, they have drawbacks in others. First, any lubricants associated with the spring are entrained away from the spring as a result of the centrifugal forces acting on the pulley during its operation. Additionally, these centrifugal forces also operate against the engagement of the spring on the hub. In other words, these centrifugal forces actually tend to force the spring out of engagement with the hub. Moreover, the bent ends or tabs of the springs further complicate the design and introduce stress concentrations into the spring which can lead to premature failure.
In view of the foregoing limitations and shortcomings of the prior art devices, as well as other disadvantages not specifically mentioned above, it should be apparent that there still exists a need for an improved one-way, over-running clutch pulley.
It is therefore a primary object of this invention to fulfill that need by providing a one-way over-running clutch pulley which overcomes the limitations and shortcomings of the prior art. Such a clutch pulley would find particular applicability with the accessory drive system of an automobile where it would allow for the relative slip between the pulley and the accessory input shaft.
It is also an object of the present invention to provide a one-way, over-running clutch pulley which accommodates large decelerations of the belt so as to reduce or eliminate the belt squeal and wear that normally occur as a result of the belt slipping relative to the pulley.
Another object of this invention is to provide an over-running clutch pulley which utilizes centrifugal forces in a positive manner during operation so as to enhance the transfer of torque to the accessory input shaft.
Still another object of this invention is to provide a one-way, over-running clutch pulley incorporating an internal coil spring to provide for the limited slip in the clutch pulley.
A related object of this invention is to provide a one-way, over-running clutch pulley which exhibits enhanced heat dissipating capabilities.
Yet another object of this invention is to provide a one-way, over-running clutch pulley having features which assist in maintaining functional operability of the pulley even in the event where the coil spring has failed or broken.